Adhesive and sealant compositions based on acrylate and methacrylate monomers polymerizable by free radical initiation are known in the art. Likewise, anaerobic compositions are known in the art. (See for example U.S. Pat. Nos. 2,895,950, 3,043,820, and 3,218,305). Anaerobic compositions are characterized by their ability to remain liquid in the presence of air but cure to a strong adhesive bond when air is excluded, as by assembling a mated nut and bolt to which the composition has been applied.
While serving many useful purposes, both standard acrylic compositions as well as anaerobic-type acrylic compositions have been limited in applicability due to degradation of adhesive strength at elevated temperatures, e.g., 250.degree. F. or more.
Among the various solutions which have been offered in the art to improve the thermal stability of cured acrylic monomer compositions generally and cured anaerobic compositions in particular, has been the teaching in U.S. Pat. No. 3,988,299 to employ certain maleimide and bismaleimide additive compounds of the formula: ##STR2## where R and R' are selected from the group consisting of alkyl, cycloalkyl, aralkyl, and alkaryl. The most preferred additive of this type is m-phenylene dimaleimide, sold under the trade name HVA-2.
While the compositions of U.S. Pat. No. 3,988,299 have proved very useful, they do have significant limitations. In particular, the imide additive has a very low solubility in most conventional acrylic monomers. As it is generally desirable to have an imide content higher than the solubility limit in order to see significant improvement in thermal stability properties, it is very difficult to synthesize formulations for commercial use which do not have severe settling problems. Unless thoroughly and uniformly mixed before use such compositions can display erratic and unpredictable cured thermal stability. Moreover, the presence of suspended particulate imide in the formulation may deleteriously effect the strength of ambient temperature cured compositions prior to thermal aging. There is, therefore, a need for an improved thermal additive for curable acrylic compositions which is substantially miscible in conventional acrylic monomers and which has improved thermal aging properties similar to those of the maleimide additives disclosed in U.S. Pat. No. 3,988,299.